(i) Technical Field
The present invention relates to a color processing apparatus, a color processing method, and a non-transitory computer readable medium.
(ii) Related Art
Input/output characteristics of an output device are obtained from a pair of color signals, namely, a color signal given to the output device and a color signal produced by measuring the color output from the output device in response to the color signal given to the output device. For example, CMY color signals or CMYK color signals are given to an image forming device that forms an image using colorants of cyan (C), magenta (M), and yellow (Y) or using cyan (C), magenta (M), yellow (Y), and black (K), and the colors of an image formed by the image forming device are measured to obtain L*a*b* color signals. The input/output characteristics from the CMY color signals or CMYK color signals to the L*a*b* color signals are obtained from the pairs of CMY color signals or CMYK color signals and L*a*b* color signals. Further, input/output characteristics of a display device are obtained as follows. Red/green/blue (RGB) color signals are given to the display device, and the colors displayed on the display device are measured to obtain L*a*b* color signals. Thus, the input/output characteristics from the RGB color signals to the L*a*b* color signals are obtained from the pairs of RGB color signals and L*a*b* color signals.
With the use of an inverse function of a function representing the above input/output characteristics, for example, CMY color signals or CMYK color signals, RGB color signals, or the like are obtained from given L*a*b* color signals. In computer color matching (CCM) known in the field of color engineering, inverse functions are solved using Newton's method.
In Newton's method, if a color signal given to an output device is represented by an input color signal and a signal of a color (for example, a color signal produced by measuring a color) output from the output device is represented by an output color signal, an appropriate initial value is given to the input color signal with respect to the given output color signal, and the input color signal is corrected on the basis of the sensitivity of the function representing the input/output characteristics of the output device, resulting in convergence. Using this principle, the input color signal is calculated from the output color signal, that is, the inverse function of the function representing the input/output characteristics is solved.
Here, an update value for the input color signal is calculated to correct the input color signal. The update value is calculated using the inverse matrix of the sensitivity matrix (Jacobian matrix). Since the inverse matrix of the sensitivity matrix is used, the solution is divergent if the sensitivity matrix is irregular. If the solution is divergent, the input color signal is not calculated because of no convergence. Therefore, it is desirable to prevent the solution from being divergent.